1. Field of the Invention
This invention relates broadly to telecommunications, the Synchronous Optical Network (SONET) and the Synchronous Digital Hierarchy (SDH). More particularly, this invention relates to a combined hardware and software implementation of the link capacity adjustment scheme (LCAS) in SONET/SDH virtual concatenation (VCAT).
2. State of the Art
The Synchronous Optical Network (SONET) or the Synchronous Digital Hierarchy (SDH), as it is known in Europe, is a common telecommunications transport scheme which is designed to accommodate both DS-1 (T1) and E1 traffic as well as multiples (DS-3 and E-3) thereof. A DS-1 signal consists of up to twenty-four time division multiplexed DS-0 signals plus an overhead bit. Each DS-0 signal is a 64 kb/s signal and is the smallest allocation of bandwidth in the digital network, i.e. sufficient for a single telephone connection. An E1 signal consists of up to thirty-two time division multiplexed DS-0 signals with at least one of the DS-0s carrying overhead information.
Developed in the early 1980s, SONET has a base (STS-1) rate of 51.84 Mbit/sec in North America. The STS-1 signal can accommodate 28 DS-1 signals or 21 E1 signals or a combination of both. The basic STS-1 signal has a frame length of 125 microseconds (8,000 frames per second) and is organized as a frame of 810 octets (9 rows by 90 byte-wide columns). It will be appreciated that 8,000 frames*810 octets per frame*8 bits per octet=51.84 Mbit/sec. The frame includes the synchronous payload envelope (SPE) or virtual container (VC) as it is known in Europe, as well as transport overhead. Transport overhead is contained in the first three columns (27 bytes) and the SPE/VC occupies the remaining 87 columns.
In Europe, the base (STM-1) rate is 155.520 Mbit/sec, equivalent to the North American STS-3 rate (3*51.84=155.520). The STS-3 (STM-1) signals can accommodate 3 DS-3 signals or 63 E1 signals or 84 DS-1 signals, or a combination of them. The STS-12 (STM-4) signals are 622.080 Mbps and can accommodate 12 DS-3 signals, etc. The STS-48 (STM-16) signals are 2,488.320 Mbps and can accommodate 48 DS-3 signals, etc. The highest defined STS signal, the STS-768 (STM-256), is nearly 40 Gbps (gigabits per second). The abbreviation STS stands for Synchronous Transport Signal and the abbreviation STM stands for Synchronous Transport Module. STS-n signals are also referred to as Optical Carrier (OC-n) signals when transported optically rather than electrically.
To facilitate the transport of lower-rate digital signals, the SONET standard uses sub-STS payload mappings, referred to as Virtual Tributary (VT) structures. (The ITU calls these structures Tributary Units or TUs.) This mapping divides the SPE (VC) frame into seven equal-sized sub-frames or VT (TU) groups with twelve columns of nine rows (108 bytes) in each. Four virtual tributary sizes are defined as follows.
VT1.5 has a data transmission rate of 1.728 Mb/s and accommodates a DS1 signal with overhead. The VT1.5 tributary occupies three columns of nine rows, i.e. 27 bytes. Thus, each VT Group can accommodate four VT1.5 tributaries.
VT2 has a data transmission rate of 2.304 Mb/s and accommodates a CEPT-1 (E1) signal with overhead. The VT2 tributary occupies four columns of nine rows, i.e. 36 bytes. Thus, each VT Group can accommodate three VT2 tributaries.
VT3 has a data transmission rate of 3.456 Mb/s) and accommodates a DS1C (T2) signal with overhead. The VT3 tributary occupies six columns of nine rows, i.e. 54 bytes. Thus, each VT Group can accommodate two VT3 tributaries.
VT6 has a data transmission rate of 6.912 Mb/s and accommodates a DS2 signal with overhead. The VT6 tributary occupies twelve columns of nine rows, i.e. 108 bytes. Thus, each VT Group can accommodate one VT6 tributary.
As those skilled in the art will appreciate, the original SONET/SDH scheme as well as the VT mapping schemes were designed to carry known and potentially foreseeable TDM (time division multiplexed) signals. In the early 1980s these TDM signals were essentially multiplexed telephone lines, each having the (now considered) relatively small bandwidth of 56-64 kbps. At that time, there was no real standard for data communication. There were many different schemes for local area networking and the wide area network which eventually became known as the Internet was based on a “56 kbps backbone”. Since then, Ethernet has become the standard for local area networking. Today Ethernet is available in four bandwidths: the original 10 Mbps system, 100 Mbps Fast Ethernet (IEEE 802.3u), 1,000 Mbps Gigabit Ethernet (IEEE 802.3z/802.3ab), and 10 Gigabit Ethernet (IEEE 802.3ae).
In recent years it has been recognized that SONET/SDH is the most practical way to link high speed Ethernet networks over a wide area. Unfortunately, the various Ethernet transmission rates (10 Mbps, 100 Mbps, 1,000 Mbps, and 10,000 Mbps) do not map well into the SONET/SDH frame. For example, the original 10 Mbps Ethernet signal is too large for a VT-6 tributary (6.912 Mbps) but too small for an entire STS-1 (51.84 Mbps) path. In other words, under the existing SONET/SDH schemes, in order to transport a 10 Mbps Ethernet signal, an entire STS-1 path must be used, thereby wasting a significant amount of bandwidth. Similar results occur when attempting to map the faster Ethernet signals into STS signals.
In order to provide a scheme for efficiently mapping Ethernet signals (as well as other signals such as Fiber Channel and ESCON) into a SONET/SDH frame, the Virtual Concatenation (VCAT) Protocol was created and has been endorsed by the ITU as the G.707 standard (ITUT-T Rec. G.707/Y.1322 (December 2003)) which is hereby incorporated by reference herein in its entirety. Similar to inverse multiplexing, Virtual Concatenation combines multiple links (members) into one Virtual Concatenation Group (VCG), enabling the carrier to optimize the SDH/SONET links for Ethernet traffic. For example, using virtual concatenation, five VT-2 (2 Mbps) links can be combined to carry a 10 Mbps Ethernet signal, resulting in full utilization of allotted bandwidth. Two STS-1 (51 Mbps) links can be combined to carry a 100 Mbps Ethernet signal, etc. Virtual Concatenation uses SONET/SDH overhead bytes to indicate a control packet. In the source to sink direction, the control packet includes: multiframe indicator (MFI), sequence number (SQ), control field (CTRL), and group identification bit (GID). MFI is actually presented in two parts (MFI-1 and MFI-2). CTRL includes information to synchronize sink to source and to provide the status of an individual group member.
Part of the emerging Virtual Concatenation Protocol includes methods for dynamically scaling the available bandwidth in a SONET/SDH signal. These methods are known as the Link Capacity Adjustment Scheme or LCAS. LCAS is a powerful network management tool because customer bandwidth requirements change over time. One simple example is a network user who, during business hours, needs only enough bandwidth to support electronic mail and worldwide web access. During non-working hours, however, the same network user may wish to conduct relatively large data transfers from one location to another to backup daily transactions, for example. It would be desirable to alter the user's available bandwidth as needed. LCAS provides a means to do this without disturbing other traffic on the link. LCAS has been endorsed by the ITU as the G.7042 standard (ITU-T Rec. G.7042/Y.1305 (February 2004)) which is hereby incorporated by reference herein in its entirety.
While Virtual Concatenation is a simple labeling protocol, LCAS requires a two-way handshake (using seven of the sixteen H4 bytes for high order STS-1 signals, and seventeen of the thirty-two K4 bits for low order VT1.5 signals). Status messages are continually exchanged and actions are taken based on the content of the messages. For example, to provide high order (STS-1) virtual concatenation, each STS-1 signal carries one of six LCAS control commands which are described as follows:
“Fixed”—LCAS not supported on this STS-1;
“Add”—Request to add this STS-1 to a VCG, thereby increasing the bandwidth of an existing VCG or creating a new VCG;
“Norm”—This STS-1 is in use;
“EOS”—This STS-1 is in use and is the last STS-1 of this VCG, i.e. the payload carrying STS-1 with the highest SQ number;
“Idle”—This STS-1 is not part of a VCG or is about to be removed from a VCG; and
“Do not use”—This STS-1 is supposed to be part of a VCG, but does not transport payload due to a broken link reported by the destination. Members of a VCG which do not carry payload are termed “inactive” whereas members which carry payload are termed “active”.
The handshaking protocol of the LCAS standard can be relatively slow and is not subject to stringent timing requirements. Once the handshaking is complete, however, the mapper device at the end node must add or remove one or more members from the affected VCG before the start of the next frame. It is not difficult to implement LCAS in hardware in order to meet this stringent timing requirement. However, LCAS is still an evolving protocol and it is difficult or impossible to alter the hardware in order to adapt to changes in the LCAS protocol. Moreover, the implementation of additional alarm and monitoring features is impossible if the implementation is entirely hardware.